Bacteria are to be found everywhere dispersed in the environment and can be beneficial or else carry diseases. The latter are normally transmitted by diseased people or animals, contaminated water and food and other external sources of contamination.
It is possible to be contaminated with bacteria responsible for the development of pathologies through contact with objects or surfaces previously manipulated by people or animals that are carriers of such bacteria.
The problem of safety against bacterial proliferation has by now reached a global dimension. The development of different types of pathogenic bacteria on the surfaces of objects/products is favoured by particular environmental conditions.
Generally, microorganisms must reach certain concentrations in order to cause damage to the body, i.e. to provoke diseases. It thus appears important to prevent conditions from arising which are suitable for their development. Besides the normal rules of hygiene, by now well-established, there is an increasingly widespread need to develop materials with intrinsic antibacterial properties to be used to prepare widely used consumer products, intended in particular to enter into contact with the skin. Combining common rules of hygiene and the use of antibacterial materials enables optimal results to be achieved in terms of reducing bacterial contamination and thus the pathologies connected thereto.
Nowadays, thermoplastic polymers are the materials most widely used to manufacture products in any sector. It thus becomes important to have at our disposal products made of plastic material capable of hindering bacterial proliferation.
Historically speaking, the antibacterial properties of silver have been known since antiquity.
In fact, even though bacteria were obviously unknown, the antibacterial and curative effects of some metals such as silver were known as far back as antiquity.
Starting from these first applications we eventually arrived at the technique of combining nano-particles of silver, or of colloidal silver (liquid suspension of microscopic particles of silver) with polymeric plastic materials, in order to impart antiseptic and antibacterial properties to the latter.
However, it is well known that the prolonged use of colloidal silver or silver particles can provoke a chronic intoxication called “argyria”.
Moreover, there is ample scientific documentation regarding the side effects of colloidal silver, which can cause skin necrosis if maintained in contact with the skin for a long time.
However, there are various metals which have antimicrobial properties, for example copper, nickel and zinc. Copper and nickel, however, are subject to strong legislative restrictions because of their high toxicity.
The toxicity of the free metallic zinc is well known, as are the neurological problems it can induce. Nonetheless, zinc ion-based compounds are less toxic than metallic zinc and retain antibacterial properties.
Over the last few years numerous studies have confirmed the antimicrobial and antiviral properties of zinc ions, especially against Gram+ bacteria such as Staphylococcus Aureus, one of the most common pathogenic bacteria responsible for skin infections, and against the principal bacteria of the oral cavity, such as Streptococci and Actinomycetes.
Zinc ions in fact possess bacteriostatic properties, i.e. they inhibit the bacterial growth because they are able to penetrate into bacterial cells, blocking different biological processes that are fundamental for the survival of the bacteria themselves.
Polymers having antibacterial properties are known; they are obtained by mixing the polymer with zinc salts and the antiseptic properties thereof are due to the ions of these metals. Although it achieves the intended result, namely, that of imparting to the polymers antiseptic and antibacterial properties belonging to the metal ion, thus combating the proliferation of microbes and bacteria, mixing the polymeric macromolecule with zinc salts poses a major disadvantage.
This disadvantage is tied to the release of zinc ions over time, due to the fact that the salts of these metals are simply mixed with the polymer and not incorporated into the polymeric structure. In fact, even if the mixing is done in an optimal manner, over time there occur phenomena whereby the metal ions are released in quantities exceeding the limits allowed by applicable European legislation, which regulates the maximum limits of metals that can migrate from a polymer to the surrounding environment. With regard to the zinc ion, this legislation sets the maximum migration limit at a value of 21 ppm.
In light of the considerations set forth above, in the art there exists a need to provide polymers with antibacterial and antiseptic properties to be used to manufacture products intended to come into contact with human or animal skin, characterized by low toxicity and by a release of zinc ions that is lower than the legal limits.